All patients with severe Factor VIII-deficient hemophilia will be seen at least twice yearly in the comprehensive clinic or coagulation laboratory. At these visits, the following information will be obtained: Estimated number of pats transfusions (first visit only); Number of transfusions; and Citrate plasma collected for Factor VIII assay. All patients with severe Factor VIII-deficient hemophilia who have an inhibitor to Factor VIII will have the following information obtained: Record of all hemorrhagic episodes, therapy used, and outcome; Monthly collections of citrated plasma; and Citrated plasma samples stored at -70 degrees C for future use.